To quote Wikipedia: "The concept of a county town is ill-defined and unofficial. Some counties have their administrative bodies located elsewhere."
This could have been better worded, but Northumberland is one of those counties. The traditional county town is Alnwick, but the County Council's headquarters have been in Morpeth since 1981 and before that (from 1889 to 1981) they were in Newcastle–upon–Tyne.
When asking about county towns, you need to know whether you're looking for the traditional county town or where the administrative headquarters are. If there's any ambiguity, you should either say which one you're looking for or allow both answers.
In this case, Morpeth should have been allowed as a correct answer.
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